On Wed, 28 Nov 2012, Frank Rowand wrote:
> 3.6.7-rt18: kernel BUG at .../kernel/sched/core.c:3817!
>
> Grant reported this same problem for 3.6.5-rt15.
>
> I am seeing it on a different arm board.
>
> Here is the BUG_ON():
>
> asmlinkage void __sched preempt_schedule_irq(void)
> {
> struct thread_info *ti = current_thread_info();
>
> /* Catch callers which need to be fixed */
> BUG_ON(ti->preempt_count || !irqs_disabled());
>
> Putting in some extra printk(), the BUG_ON() is triggering because
> ti->preempt_count is non-zero.
>
>
> It appears that the cause is in arch/arm/kernel/entry-armv.S.
>
> The call to preempt_schedule_irq() is from svc_preempt:
>
> #ifdef CONFIG_PREEMPT
> svc_preempt:
> mov r8, lr
> 1: bl preempt_schedule_irq @ irq en/disable is done inside
>
>
> svc_preempt is branched to from one of two possible places. The first was
> present before the lazy preempt code was added. The first appears ok to me.
> (Note that the first branch does not occur if preempt count is non-zero.)
>
> The second branch can occur even if the preempt count is non-zero (which is
> what the BUG_ON() is finding):
>
> __irq_svc:
> svc_entry
> irq_handler
>
> #ifdef CONFIG_PREEMPT
> get_thread_info tsk
> ldr r8, [tsk, #TI_PREEMPT] @ get preempt count
> ldr r0, [tsk, #TI_FLAGS] @ get flags
> teq r8, #0 @ if preempt count != 0
> movne r0, #0 @ force flags to 0
> tst r0, #_TIF_NEED_RESCHED
> blne svc_preempt
> ldr r8, [tsk, #TI_PREEMPT_LAZY] @ get preempt lazy count
> ldr r0, [tsk, #TI_FLAGS] @ get flags
> teq r8, #0 @ if preempt lazy count != 0
> movne r0, #0 @ force flags to 0
> tst r0, #_TIF_NEED_RESCHED_LAZY
> blne svc_preempt
> #endif
Bah. I knew that I had messed up the ASM magic.
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